1. Field of the Invention
The invention relates generally to the utilization of dimethyl ether and the production of hydrogen and carbon oxides. More specifically, the invention relates to a process for catalytically reacting dimethyl ether in the presence of steam.
2. Description of the Prior Art
The conversion of low molecular weight alkanes, such as methane, to synthetic fuels or chemicals has received increasing attention as low molecular weight alkanes are generally available from secure and reliable sources. For example, natural gas wells and oil wells currently produce vast quantities of methane. In addition, low molecular weight alkanes are generally present in coal deposits and may be formed during mining operations, in petroleum processes, and in the gasification or liquefaction of coal, tar sands, oil shale, and biomass.
Many of these alkane sources are located in relatively remote areas, far from potential users. Accessibility is a major obstacle to effective and extensive use of remotely situated methane, ethane and natural gas. Costs associated with liquefying natural gas by compression or, alternatively, constructing and maintaining pipelines to transport natural gas to users are often prohibitive. Consequently, methods for converting low molecular weight alkanes to more easily transportable liquid fuels and chemical feedstocks are desired and a number of such methods have been reported.
The reported methods can be conveniently categorized as direct oxidation routes or as indirect syngas routes. The direct oxidative routes convert lower alkanes to products such as methanol, gasoline, and relatively higher molecular weight alkanes. In contrast, the indirect syngas routes involve the production of synthesis gas as an intermediate product.
Routes are known for converting methane to dimethyl ether. For example, methane is steam reformed to produce synthesis gas. Thereafter, dimethyl ether and methanol can be manufactured simultaneously from the synthesis gas, as described in U.S. Pat. No. 4,341,069 issued to Bell et al. The '069 patent recommends a dimethyl ether synthesis catalyst having copper, zinc, and chromium co-precipitated on a gamma-alumina base. Interestingly, the '069 patent states that dimethyl ether product can be charged as power generator fuel to a combustor of a gas turbine-prime mover arrangement, either alone or simultaneously with synthesis gas.
Alternatively, methane is converted to methanol and dimethyl ether is subsequently manufactured from methanol by passing a mixed vapor containing methanol and water over an alumina catalyst, as described in an article by Hutchings in New Scientist (3 Jul. 1986) 35.
Having a relatively low vapor pressure, dimethyl ether is readily transportable. Moreover, dimethyl ether can be economically produced in relatively small quantities, as compared to materials such as compressed natural gas which require economies of scale associated with large cryogenic plants to be produced competitively. On the other hand, synthesis gas reportedly produces very little atmospheric pollution when combusted with air as fuel. Therefore, a practical process for converting dimethyl ether to synthesis gas on a commercial scale would be attractive to, among others, natural gas producers situated far from fuel consumers.
Known processes for producing synthesis gas typically react hydrocarbons with steam at elevated temperature over a catalyst. Generally, more complex hydrocarbons are converted to methane which is then steam reformed to produce hydrogen or synthesis gas.
United Kingdom Patent Application GB 9913496 A listing Lywood as inventor describes the production of hydrogen-containing gas streams by an endothermic catalyzed reforming between methane and steam. The '496 application proposes the following equations for the steam reforming of methane: EQU CH.sub.4 +H.sub.2 O.fwdarw.CO+3H.sub.2 1. EQU CH.sub.4 +2H.sub.2 O.fwdarw.CO.sub.2 +4H.sub.2 2. EQU CH.sub.4 +CO.sub.2 .fwdarw.2CO+2H.sub.2 3.
U.S. Pat. No. 4,592,903 issued to Osman et al., states that carbon monoxide can be endothermically converted to carbon dioxide and hydrogen through a reaction termed a water-gas shift, represented by the equation: EQU CO+H.sub.2 O.fwdarw.CO.sub.2 +H.sub.2 4.
Reportedly, the "shift" reaction, can be accomplished in two shift conversion vessels operating at different temperatures to maximize yield. The '903 patent states that a temperature of from about 600 to 900 degrees F. and a pressure of about 300 to 1,000 psig is effective in a high-temperature shift converter containing a supported, chromium-promoted iron catalyst. The '903 patent further states that a low-temperature shift conversion takes place over a catalyst comprising a mixture of zinc and copper oxides at a temperature of from about 400 to 500 degrees F. and a pressure of from about 300 to about 1,000 psig.
It is important to distinguish between the steam reforming of hydrocarbons, as described above, and the partial oxidation of hydrocarbons. The partial oxidation of methane produces two moles of diatomic hydrogen for each mole of methane reacted. In contrast, the steam reforming of methane produces three moles of diatomic hydrogen per mole of reacted methane.
The partial oxidation of methane is described, for example, in U.S. Pat. No. 4,618,451 issued to Gent. The '451 patent states that methane is reacted with oxygen from an air separation plant, the proportion of oxygen being less than sufficient for complete combustion. A hot gas containing hydrogen and carbon monoxide is said to be produced. The '451 patent also states that steam or nitrogen can be present during the combustion to act as a temperature modifier and to avoid soot formation. Additional hydrocarbon is reportedly injected into the hot gas, and the resulting gas mixture is reacted over a steam reforming catalyst.
A particular class of partial oxidation processes for converting methane or natural gas to synthesis gas are known as autothermal processes. By convention, the autothermal process includes an exothermic oxidation step and an endothermic steam reforming step which are in approximate heat balance. For example, U.S. Pat. No. 5,112,257 issued to Kobylinski and assigned to the assignee of the present invention, describes an autothermal process for converting natural gas to synthesis gas which includes the steps of mixing natural gas with air, subjecting a resulting mixture to simultaneous partial oxidation and steam reforming reactions, and subsequently reacting unconverted alkanes with water in the presence of a catalyst having steam reforming activity.
Processes which produce hydrogen or hydrogen-containing mixtures by reacting a single-carbon saturated alcohol, methanol, with steam are collectively termed methanol steam reforming processes. U.S. Pat. No. 4,091,086 issued to Hindin et al. describes a process for producing hydrogen by reacting steam with methanol in the presence of a catalytic composition at elevated temperatures. The '086 patent reports states that methanol can be converted to hydrogen in a single-stage reaction over a catalytic composition comprising zinc oxide, copper oxide, thorium oxide, and aluminum oxide. Moreover, the '086 patent states, without citing authority or presenting evidence in support, that the composition catalyzes a purported methanol decomposition. The purported decomposition is described as producing significant amounts of carbon monoxide which are immediately consumed in a water gas shift reaction.
U.S. Pat. No. 4,743,576 issued to Schneider et al. describes a catalyst for the production of synthesis gas or hydrogen from aqueous methanol by dissociation or steam reforming. The catalyst reportedly contains a noble metal component on an oxide carrier which comprises an oxide of cerium or titanium and, also, an oxide of zirconium or lanthanum.
U.S. Pat. No. 4,865,624 issued to Okada describes a process for reacting methanol with steam including a decomposition reaction zone regulated at a temperature between 250 and 300 degrees C. and a conversion reaction zone regulated between 150 and 200 degrees. The '624 patent postulates an alleged methanol decomposition for producing hydrogen and carbon monoxide directly from methanol. The conversion reaction zone described in the '624 patent is apparently intended to promote the well-known water gas shift reaction.
An integrated turbo-electric power generation system which incorporates methanol reforming as a source of fuel and as a means of heat recovery is described in sales literature circulated by the New Energy and Industrial Technology Development Organization under the authority of the Ministry of International Trade and Industry of Japan cerca 1985. The sales literature states that methanol and steam are passed through catalysts at temperatures in the range of 250 to 350 degrees C. to produce hydrogen and carbon dioxide in an endothermic reaction. The hydrogen-containing gas is reportedly combusted with air to drive a turbine. The sales literature indicates that the reforming reactor charge and the combustion air stream can be heat exchanged with the turbine exhaust to promote energy efficiency.
Despite some earlier speculation regarding the existence of a direct methanol decomposition mechanism, practitioners generally agree that methanol steam reforming proceeds by a mechanism which does not involve the direct decomposition of methanol to hydrogen and carbon monoxide. Rather, it is accepted that the stem reforming of methanol creates methyl format and formic acid as intermediaries. For example, an article by Jiang et al., Applied Catalysis A: General, 97 (1993) 145-158 Elsevier Science Publishers B.V., Amsterdam, cites studies and presents experimental data indicating that stem reforming of methanol proceeds via dehydrogenation to methyl formate, hydrolysis of methyl formate to formic acid, and decomposition of formic acid to carbon dioxide and hydrogen. According to the Jiang et al. article, no carbon monoxide production was detected while passing methanol over a copper, zinc oxide and alumina catalyst at temperatures below 250 degrees C. The Jiang et al. article reports that significant amounts of carbon monoxide were formed only at temperatures over 300 degrees C. Moreover, the Jiang et al. article states that methanol steam reforming proceeds in accord with the following equations: EQU 2CH.sub.3 OH.fwdarw.CH.sub.3 OCHO+2H.sub.2 5. EQU CH.sub.3 OCHO+H.sub.2 O.fwdarw.CH.sub.3 OH+HCOOH 6. EQU HCOOH.fwdarw.CO.sub.2 +H.sub.2 7.
UK Patent Application GB 2085314 A listing Twigg as inventor describes a catalytic process for reacting a hydrocarbon with steam in net endothermic conditions to produce a gas containing carbon oxides and hydrogen. The process is reportedly carried out using a catalyst comprising the product of thermally decomposing and reducing intimately associated compounds of nickel and/or cobalt and at least one difficultly reducible metal. Reportedly, the catalyst also comprises a water-insoluble compound of an alkali metal oxide with an acidic or amphoteric oxide or mixed oxide.
The '314 application states that the alkali metal, usually sodium or potassium, is chosen on the basis of the vapor pressure of its hydroxide form, so as to be available as an alkaline metal hydroxide to catalyze a reaction between carbon deposited on the catalyst and steam. The '314 Application speculates that the starting hydrocarbon can be any of those proposed for use with a catalytic steam/hydrocarbon reaction including methane, natural gas, liquified petroleum gas, naphtha, methanol, dimethyl ether, and isobutyraldehyde. However, as explained below the presence of potassium hydroxide actually hinders the reaction of dimethyl ether and steam.
In order to better utilize remotely situated sources of natural gas, to transport the energy inherent in natural gas in a safer and more economic manner, and to provide a fuel which creates very little atmospheric pollution when combusted in air, a commercially practical method for transforming dimethyl ether and steam to synthesis gas is desired. Preferably, the improved method is suitable for integration into modern power generation schemes.